Hitherto, as a punching unit of the sort described above, there have been known a dedicated type punching unit dedicated for punching two holes, three holes or four holes and a switching type punching unit arranged so as to be able to commonly use punches in punching two holes and three holes or two holes and four holes. As the switching type punching unit, there is known a sliding-cam type punching unit wherein a stroke range of a cam is divided into two stages so as to be able to punch two holes in a certain stroke range and to punch three holes in another stroke range as disclosed in Japanese Patent Laid-Open No. 2001-198889.